1. Field of the Invention
The present invention relates to a method of substantially precluding the accumulation of combustible organic vapors in a storage container.
2. Description of the Prior Art
When volatile organic materials are maintained in a confined storage container, a hazardous situation exists if the concentration of the combustible organic vapor in the air in the storage container falls within the explosive envelope or range. By the term "explosive envelope" is meant the vapor/air ratio range that will support combustion. The vapors will not explode if the atmosphere in the storage container is overrich, which means that there is an excess of vapors and, consequently, there is insufficient air to support combustion, i.e., the vapor/air ratio is above the upper limit of the explosive envelope. The vapors also will not explode if the atmosphere in the storage container is too lean which means there is not enough vapor to support combustion, i.e., the vapor/air ratio is below the lower limit of the explosive envelope.
Petroleum tankers carry crude oil as it is extracted from the ground and such crude oil normally contains considerable quantities of volatile hydrocarbons. There is no safety problem when the storage tanks of the petroleum tankers are completely filled or "topped off" with crude oil, since it is not the oil itself that is highly combustible or explosive under these conditions but rather the hydrocarbon vapors given off therefrom.
A measure of the flammability of combustible vapors is the "flash point" which is defined as the lowest temperature at which enough vapors are given off to form a flammable mixture of vapors and air immediately above the liquid surface. The flash points of various solvents differ widely. Flash points are affected by temperature, barometric pressure and molecular structure. Hydrocarbon vapors have low flash points and burn or explode at temperature as low as -40.degree. F.
After a tanker has discharged its crude oil cargo, pools of thick crude oil residue often remain in the bottom of the tanks out of reach of the main discharge pumps and the walls of the holding tanks are oftimes covered by a thin layer of an oil-wax mixture. The residue of crude oil in the hold of the ship desorbs the volatile hydrocarbons into the empty tank chamber and an explosive mixture of vapor and air may result which can be ignited by a spark. To prevent this danger, tanker crews generally scrub down the walls of the container in the ship and manually remove the residue therefrom. The crude residue from the tanks is normally discarded overboard and for many years constituted a major source of pollution at or near the harbors where such cargoes are discharged. Manual removal of the oil dregs is also a very expensive procedure.
A convenient way of eliminating the hazardous condition and environmental pollution inherent in the existing practice is to control the atmosphere inside the tank so as to ensure that the vapor/air mixture remains outside of the explosive envelope even in the presence of residual crude oil.